1. Field of the Invention
This invention relates to a process for treating an aqueous waste stream from a chemical production plant, more particularly an aqueous waste stream which contains at least one alkali metal carboxylate containing at least 3 carbon atoms, to produce a solution of an alkali metal hydroxide for recycle to the chemical production plant. In addition, the invention relates to a waste treatment plant for carrying out such a process.
2. Description of the Related Art
A number of types of chemical production plants produce a waste stream which contains at least one alkali metal carboxylate containing at least 3 carbon atoms up to about 22 carbon atoms. Normally, such an alkali metal carboxylate is a salt of an aliphatic acid. Examples of such chemical production plants include aldolisation plants in which a saturated aliphatic aldehyde, such as n-butyraldehyde or n-valeraldehyde, is converted by aldolisation followed by dehydration to an unsaturated aldehyde containing twice as many carbon atoms as the starting aldehyde, for example, 2-ethylhex-2-enal from n-butyraldehyde or 2-propylhept-2-enal from n-valeraldehyde. In such a process, the starting aldehyde may contain also a minor amount of the corresponding isomeric aidehyde, for example, iso-butyraldehyde or, in the case of n-valeraldehyde, also iso-valeraldehyde (2-methylbutyraldehyde) and 3-methylbutyraldehyde. The aldolisation-dehydration products, such as 2-ethylhex-2-enal and 2-propylhept-2-enal, find use as intermediates in the production of important plasticiser alcohols, such as 2-ethylhexanol and 2-propylheptanol.
An aldolisation-dehydration process is typically conducted in the presence of an aqueous solution of an alkali metal hydroxide as catalyst. When the starting aldehyde is n-butyraldehyde the reaction proceeds as follows: ##STR1##
A competing reaction is the Cannizzaro reaction which yields a mixture of the alcohol corresponding to the starting aldehyde and an alkali metal salt of the corresponding carboxylic acid. Thus, if the catalyst is sodium hydroxide, the Cannizzaro reaction is: EQU 2CH.sub.3 CH.sub.2 CH.sub.2 CHO+NaOH.fwdarw.CH.sub.3 CH.sub.2 CH.sub.2 COONa+CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.2 OH.
In order to limit the build up of the unwanted sodium carboxylate in the plant it is usual to purge a part of the aqueous liquor as a waste stream from the aldolisation plant. However, since this is alkaline and has a significant organic content, it cannot be discharged directly to the environment. Normally, the waste stream is neutralised and then subjected to appropriate biological treatment in order to reduce the oxygen demand of the waste stream to acceptable levels. The cost and inconvenience of importing to the plant site the acid used to neutralise the waste stream and the make up alkali needed to replace the alkali metal ions removed in the waste stream are drawbacks to the existing methods of operating an aldolisation plant. In addition, the capital cost of the necessary treatment plant required to reduce the oxygen demand to an acceptable level represents an undesirable additional expense, particularly since with increasing pressure to avoid pollution, plant operators are coming under increasing pressure to reduce the oxygen demand still further which requires even more capital expenditure.
Another process which produces an aqueous waste stream containing an alkali metal carboxylate is the conversion of iso-butyraldehyde to neopentyl glycol by reaction with formaldehyde. This proceeds by aldol condensation of formaldehyde with iso-butyraldehyde followed by a cross-Cannizzaro reaction between the intermediate .beta.-hydroxyaldehyde, 2,2-dimethyl-3-hydroxypropanal, and formaldehyde according to the following equations: ##STR2##
In this case, the aqueous waste stream contains sodium formate; in addition, it contains sodium iso-butyrate formed as a result of a Cannizzaro reaction from iso-butyraldehyde according to one of the following reactions: ##STR3##
Another sodium salt in the waste liquor is sodium hydroxypivalate. This is formed by a Cannizzaro reaction of the intermediate .beta.-hydroxyaldehyde, 2,2-dimethyl-3-hydroxypropanal, according to the following equation: ##STR4##
or according to the following equation: ##STR5##
Again, this process is typically conducted using an alkali metal hydroxide or carbonate solution as catalyst.
Neopentyl glycol finds application in a range of technologies, including waterborne and alkyd surface coatings, gel coatings for fibreglass-reinforced plastics, powder coatings, lube oil additives, plasticisers and polyurethanes; the aldol product is produced without any dehydration step.
1,1,1-trimethylol propane is also of value, inter alia, in the production of alkyl resin coatings and can be produced by the aldol condensation of formaldehyde with n-butyraldehyde followed by hydrogenation. The aldolisation proceeds according to the following equations: EQU CH.sub.3 CH.sub.2 CH.sub.2 CHO+HCHO.fwdarw.CH.sub.3 CH.sub.2 CH (CHO)CH.sub.2 OH
and EQU CH.sub.3 CH.sub.2 CH(CHO)CH.sub.2 OH+HCHO.fwdarw.CH.sub.3 CH.sub.2 C(CHO)(CH.sub.2 OH).sub.2.
These reactions can be catalysed by a solution of an alkali metal hydroxide, such as sodium hydroxide. The hydrogenation reaction is: EQU CH.sub.3 CH.sub.2 C(CHO)(CH.sub.2 OH).sub.2 +H.sub.2.fwdarw.CH.sub.3 CH.sub.2 C(CH.sub.2 OH).sub.3.
In a manner that is analogous to by-product formation in the synthesis of neopentyl glycol, there can be formed as byproducts by alkali-consuming side reactions, sodium butyrate, sodium formate and sodium 2,2-di(hydroxymethyl)-butyrate.
The conversion of 2,2-di(hydroxymethyl)-butyraldehyde to 1,1,1-trimethylol propane can be effected by a crossed Cannizzaro reaction using formaldehyde as the reducing agent in the presence of a basic catalyst. The reaction involved is: EQU CH.sub.3 CH.sub.2 C(CHO)(CH.sub.2 OH).sub.2 +HCHO+NaOH.fwdarw.CH.sub.3 CH.sub.2 C(CH.sub.2 OH).sub.3 +HCOONa.
In this case, at least one mole of sodium formate is produced per mole of neopentyl glycol produced.
Another important commercial chemical that finds its principal application in the surface coating industry as a raw material for oil-modified alkyd resins and synthetic drying oils is pentaerythritol. This can be produced by successive aldolisation steps followed by a hydrogenation or cross-Cannizzaro step, the starting materials being formaldehyde and acetaldehyde. The reactions involved are: EQU CH.sub.3 CHO+HCHO.fwdarw.HOCH.sub.2 CH.sub.2 CHO; EQU HOCH.sub.2 CH.sub.2 CHO+HCHO.fwdarw.(HOCH.sub.2).sub.2 CH--CHO;
and EQU (HOCH.sub.2).sub.2 CH--CHO+HCHO.fwdarw.(HOCH.sub.2).sub.3 C--CHO.
There then follows either: EQU (HOCH.sub.2).sub.3 CH--CHO+HCHO+NaOH.fwdarw.(HOCH.sub.2).sub.4 C+HCOONa;
or EQU (HOCH.sub.2).sub.3 CH--CHO+H.sub.2.fwdarw.(HOCH.sub.2).sub.4 C.
A still further important commercial chemical is 2,2,4-trimethyl-1,3-pentanediol which is used, inter alia, as an intermediate in the production of unsaturated polyesters. This can be produced by aldolisation (or, as it may alternatively be termed, aldo-trimerisation) of iso-butyraldehyde followed by hydrogenation of the resulting intermediate aldolisation product, 2,6-di-iso-propyl-5,5-dimethyl-1,3-dioxan-4-ol, according to the following equations: ##STR6##
Certain hydrocarbon streams, for example C.sub.10 to C.sub.12 /C.sub.13 olefin streams produced by the reaction of carbon monoxide and hydrogen according to the Fischer-Tropsch process, contain oxygenated materials such as carboxylic acids and phenolic materials. These acids and phenolic materials can be removed by washing with dilute aqueous alkali.
Another industrial process which produces an alkaline waste stream is esterification. Organic carboxylic acid ester streams produced in commercial esterification plants are often washed with aqueous alkali in order to remove any unreacted organic carboxylic acids.
Production of all of the above mentioned commercial chemicals results in formation of an alkaline waste stream containing at least one alkali metal carboxylate, usually a sodium carboxylate. In each case, neutralisation and biological treatment are the methods of choice in order to render the waste stream fit for discharge to the environment. Nevertheless, the waste stream will still contain sodium or other alkali metal values which may not always be acceptable, for example, if it is intended that the treated waste stream is to be used for irrigation of crops. Moreover, in each case, it is necessary to import acid to the plant for the purpose of neutralisation and also make up quantities of sodium hydroxide or carbonate in order to replace the sodium values lost in the waste liquor.
An industrial process which produces an alkaline waste stream containing organic matter is the so-called Kraft process which is widely used in the wood pulp industry. In a typical procedure for treating such a waste stream, the sodium-containing waste liquor is concentrated and combusted in a furnace which is designed for burning the organic part of the sodium salts and for reduction of sulphur-containing salts to sodium sulphide. Sodium hydroxide is recovered by treating the green liquor product from the furnace melt with lime so as to convert sodium carbonate to sodium hydroxide and calcium carbonate. For further details of such a procedure reference may be made to a paper entitled "A simple integrated liquor preparation system" by J. F. Kuehl, Proceedings of the Symposium on Recovery of Pulping Chemicals held in Helsinki, May 13 to 17, 1988, pages 523 to 551.
Although solutions containing alkali metal carboxylates are amenable to treatment by similar methods to that described by J. F. Kuehl, the drawbacks are that the sodium hydroxide so produced is generally of low purity and that the process is costly not only in terms of fuel usage but also in terms of equipment costs.
It has been suggested in "The Green Potential of Electrochemistry", Engineering Practice, November 1992, pages 132 to 141 by D. Pletcher and N. L. Weinberg to use electrolysis cells to treat effluents. This paper mentions the need for technology to convert sodium salts back into sodium hydroxide.
The use of bipolar membrane technology for recovery of acid/base values from salt streams which are routinely generated in processing operations such as metal pickling, rayon manufacture, flue gas scrubbing, and fermentation is described in a paper entitled "Aquatech Membrane Technology for Recovery of Acid/Base Values from Salt Streams" by K. N. Mani et al., Desalination, Vol. 68 (2-3), pages 149 to 166 (1988). Table I of this paper lists various technology applications to which the technique can be applied.
EP-A-0096239 and U.S. Pat. No. 4,504,373 disclose an electrodialytic water splitting process for conversion of alkali metal sulphate values derived from spent rayon spin baths.
Conversion of alkali metal salts such as sodium sulphate or sodium phosphate into useful industrial feedstocks such as sulphuric acid and phosphoric acid by electrolysis in a membrane cell is described in U.S. Pat. No. 4,561,945.
In GB-A-787976 there is taught electrolytic decomposition of salts of organic aliphatic acids using a multicompartment cell having an anode compartment, a second compartment to contain the organic aliphatic acid resulting from electrolysis and, a cathode compartment, with appropriate permselective membranes between the compartments. One of the objectives of the process is to avoid treating the salts with a mineral acid.
Various specifications describe processes for effecting electrolysis of sodium sulphate to produce sulphuric acid and sodium hydroxide including EP-A-0426649, EP-A-0449071, EP-A-0531999, EP-A-0532188, and U.S. Pat. No. 3,134,729.
Production of sodium hydroxide and ammonium sulphate from sodium sulphate by an electrolysis technique is taught in U.S. Pat. No. 5,098,532.
Splitting of sodium sulphate into sodium hydroxide and sulphuric acid using bipolar membrane electrolysis is described in a paper entitled "A Solution to Caustic/Chlorine Imbalance: Bipolar Membrane Electrodialysis" by M. Paleologou et al., Journal of Pulp and Paper Science, Vol. 18, No. 4 (July 1992), pages J138 to J145.
U.S. Pat. No. 4,041,129 teaches a process for treating an acidic feed gas, such as a refinery gas, natural gas, or cracked gas, which contains H.sub.2 S, CO.sub.2, COS, low molecular weight mercaptans or mixtures thereof. The process involves washing the acidic feed gas with a solution containing sodium hydroxide and sodium sulfate. The reaction effluent is passed through an extraction or adsorbent unit in order to remove organic impurities It then passes into a reaction chamber in which it is contacted with a solution containing sulfuric acid and sodium sulfate. The resulting treated liquid is passed to an acidic gas stripper which removes CO.sub.2 and H.sub.2 S therefrom. The liquor from the acidic gas stripper is fed to the middle compartment of a three compartment electrolytic cell. The solutions used for washing the acidic feed gas and for reaction with the effluent are taken from the relevant electrolyte compartments.
GB-A-216741 proposes oxidation of an organic compound with an alkali hypohalite in an aqueous medium, separating the oxidized organic compound from waste fluid containing alkali halogenide, subjecting the waste fluid to electrolysis to produce alkali hydroxide and halogen, reacting the alkali hydroxide with halogen to produce alkali hypohalite, and recycling the alkali hypohalite as the oxidizing agent. The oxidized compound can be a carboxylic acid when the oxidizable compound is an aldehyde.
The Derwent Abstract of JP-A-05023676 proposes treatment of a waste liquor containing ammoniacal nitrogen and/or organic acid by adjusting the pH to below 6, distilling to separate solid and liquid, and electrolysing using a platinum electrode.
U.S. Pat. No. 4,752,363 is concerned with effluent treatment where the effluent contains sodium hydroxide from, for example, a textile treatment process, as well as multivalent ions and soluble and insoluble organic and inorganic matter. The process involves pH adjustment using an acid gas, followed by filtration. The filtrate is subjected to electrolysis.
In EP-A-0631988 there is proposed a process for the purification of effluents from the aidolisation reaction characterised in that the effluent is adjusted to a pH value of 0 to 6 and an organic phase which thereby separates out is optionally separated and the effluent finally extracted with monoalcohols which contain 8 or more carbon atoms in the molecule and/or with hydrocarbons which contain more than 6 carbon atoms in the molecule.
It would be desirable to provide a process for treating these and other alkaline process waste streams containing an alkali metal carboxylate or carboxylates which avoids the consumption of an acid for neutralisation. It would further be desirable to provide a process for treating an alkaline waste liquor containing at least one alkali metal carboxylate which results in essentially no loss of alkali metal values from the process and accordingly obviates substantially the need to provide make up quantities of alkali to replace alkali metal values lost from the plant.